The invention relates to a method for mechanically disintegrating biogenic sewage sludge, for which the sewage sludge, in an initial state, contains microorganisms in the form of cells and a solid, which is formed essentially by aggregates of the cells and suspended materials, and for which a disintegrating process causes destruction of aggregates and destruction of cells.
The sewage sludge under discussion here consists of solid and liquid and is in a biological purification stage, in which organic components, which can be decomposed easily, are converted into inorganic end products such as water, carbon dioxide or nitrogen. In order to accelerate this process, microorganisms are mixed with the sewage sludge, sludge flakes or aggregates of very fine suspended material and microorganisms being formed. These aggregates comprise cells, formed by microorganisms. The effectiveness of the biological cleaning stage is improved by disintegrating the sewage sludge. The disintegration of the sewage sludge is understood to be the destruction of aggregates and cells.
In principle, the effects of a disintegration which effects can be measured, are:
the particle size distribution Q3(x) with the integral characteristic values of the particle size distribution, xe2x80x9caverage diameter xmxe2x80x9d (assessment of the coarse portion) and xe2x80x9cSauter diameter xstxe2x80x9d (assessment of the fines portion)
chemical oxygen demand (COD) gel
esterase activities EA (extra cellular hydrolytic enzymes)
concentration of volatile organic acids as acetic acid equivalents.
The following is disintegrating equipment, suitable for use on a large, industrial scale: agitated ball mill, high-pressure homogenizer, ultrasonic homogenizer, high performance pulsed equipment and the lysate centrifuge. Each disintegration device brings about a disintegration process. In the case of a method, which has become known through practice and is of the type named above, only one disintegration process is applied to the sewage slugged, primarily a destruction of aggregate being brought about and the cells being destroyed only to a slight extent. In this way, disintegration results can be achieved, which lead to an improvement in the further treatment of the disintegrated sewage sludge.
It is an object of the invention to create a method of the type named above, for which the disintegration is improved. The inventive method, which accomplishes this objective, is characterized in that primarily destruction of aggregates is brought about in a first disintegration process and primarily destruction of the cells in a subsequent, separate, second disintegration process.
Due to the use of two separate disintegration processes, a considerable improvement is achieved in the disintegration, since cells, rather than aggregates, are supplied to the second disintegration process. The increased destruction of cells brings about an improvement in the disintegration. The biogenic sewage sludge is not crude sewage sludge but one activated by microorganisms and the objective is to increase the efficiency of the microorganisms. The destruction of the cells leads to the release of carbon, which is nourishment for the microorganisms, which become increasingly effective due to the availability of the nourishment. During the first disintegration process, cells are destroyed directly only to a slight extent and enzymes, which destroy the cells, are released due to the destruction of aggregates. Cells are destroyed directly and to an appreciable extent by the second disintegration process itself. The disintegration process is a treatment process primarily only for the disintegration of sewage sludge and not for any other type of treatment.
Because of the inventive method, the amount of residual sludge, finally remaining, is decreased markedly. The inventive method follows a decomposition stage, which is aerobic or anaerobic and during which gas in produced. The decomposition stage preferably is a putrefaction basin. Because of the inventive method, the yield of gas is increased and the quality of the residual sludge, which is a decomposed organic product, is improved. In the case of the inventive method, the improvement in the disintegration is achieved by increasing the available carbon content. The carbon content of the sewage sludge is reflected by the measurable chemical oxygen demand (COD). If there is primarily destruction of aggregates in the first disintegration process, then this means that at least 60% and preferably at least 80% of aggregates, which can be detected at the inlet of the first disintegration process, is destroyed at the outlet of the first disintegration process. If there is primarily destruction of cells in the second disintegration step, then this means that at least 30% of the cells, which can be detected at the outlet of the second disintegration process, are destroyed at the outlet of the second disintegration process.
As a rule, it is difficult to measure the content of aggregates and cells in the sewage sludge directly. For this reason, it is appropriate to measure the carbon content chemical oxygen demand of the sewage sludge. It is particularly appropriate and advantageous if the carbon content is up to 200 mg/L of COD gel at the inlet of the first disintegration process, up to 400 mg/L of COD gel at the outlet of the first disintegration process or at the inlet of the second of the second disintegration process and up to 2,000 mg/L of COD gel at the outlet of the second disintegration process. Within the scope of these COD values, the inventive method proceeds in a practically advantageous manner.
The inventive method can be realized with two similar processes, two similar disintegration devices being provided in series. It is particularly appropriate and advantageous if the second disintegration process differs from the first disintegration process. The first process can be directed to the destruction of aggregates and the second to the destruction of molecules. Accordingly, the inventive method can be carried out better. Especially if the two disintegration processes are different in their nature and mode of action, there is an unexpectedly large improvement in the disintegration.
It is particularly appropriate and advantageous if the first disintegration process is of a type, in which the sewage sludge is stressed with severing parts and/or if the second disintegration process is of a type, in which the sewage sludge is stressed with energy fields. The destruction of aggregates, which are larger than cells, can be brought better with the help of severing parts; for example, agitated ball mills and lysate centrifuges work with severing parts. After the first disintegration process by means of severing parts, at least 90% and frequently up to 99% of the aggregates are destroyed. The destruction of cells, which are smaller than aggregates, can be brought about better with the help of energy fields, such as high performance pulsed equipment and ultrasonic homogenizers.
It is particularly appropriate and advantageous if, for the first disintegrating process, a rotating wing device is used and/or if, for the second disintegrating process, an ultrasonic homogenizer is used. The rotating wing device, which will be described in greater detail in the following, brings about the disintegration of aggregates in an improved manner. The ultrasonic homogenizer, which is known, brings about the disintegration of cells in an improved manner. The combined use of the rotating wing device and of the ultrasonic homogenizer is associated with a further improvement in the disintegration due to the increase in the available carbon content. The ultrasonic homogenizer is a high-performance ultrasonic device and the rotating wing device is a rotating turbulence disintegrator.
For a two-step disintegration installation with a rotating wing device and an ultrasonic homogenizer, the following carbon values are obtained:
inlet of the two-step rotating wing installationxe2x80x94100 mg/L of COD gel,
outlet of the two-step rotating wing installationxe2x80x94200 mg/L of COD gel,
inlet of the 2-step ultrasonic installationxe2x80x94200 mg/L of COD gel,
outlet of the 2-step ultrasonic installationxe2x80x941072 mg/L of COD gel,
only ultrasonic installation in a single step operationxe2x80x94COD increased from 100 mg/L to 456 mg/L.
Experiments have shown that, by combining the energy field disintegration process with the preceding severing parts disintegration process, there is a far-reaching effect in regard to the change in the particle size. The comminuting effect of the severing parts disintegration process on the sewage sludge, which can be recognized clearly in photographs, taken through a light microscope, evidently improves the disintegrating action of the energy field disintegration process. The mechanical disintegration of the sewage sludge by the severing parts leads to an increase in the specific surface area, so that an immediate effect of the energy field input on the already comminuted sludge particles can take place. The synergistic effects, which resulted from the combination of the two disintegrating processes, accordingly can be demonstrated by means of the material-releasing, enzyme-stabilizing and aggregate structure-changing effect of the input of energy the sewage sludge obtained last.
It is particularly appropriate and advantageous if the sewage sludge is heated to at least 20xc2x0 C., but not to more than 45xc2x0 C. before the first disintegration process. The disintegration can be improved appreciably by heating the heating the biogenic sewage sludge. The upper limit of the temperature is determined by the harmful effect on enzymes.